1. Field of the Invention
The invention relates to a method for forming a field oxide layer for separating device to device in a highly integrated semiconductor device, and more particularly to a method for forming a field oxide layer which eliminates the bird's beak phenomena that occurs during the manufacturing process of the field oxide layer, thereby reducing the unit area of the semiconductor device.
Generally, in order to form a field oxide layer for separating device to device according to the prior art as illustrated in FIG. 1, a substrate oxide layer 2 is formed on a silicon substrate 1. A nitride layer 3 is then deposited on the entire surface of the substrate oxide layer 2. A portion of the nitride layer 3 is removed by a mask pattern process and an etching process in order to expose a portion of the substrate oxide layer 2, thereby defining active regions 1X, 1Y on the substrate 1 which underlies the remaining nitride layer 3. Thereafter, an impurity is implanted into the exposed portion of the substrate oxide layer 2. The exposed substrate oxide layer 2 is then oxidized by a LOCOS (Local Oxidation of Silicon) process, so that a field oxide layer 4 is formed both on the substrate oxide layer 2 and into the silicon substrate 1.
However, in the prior art described above, when the field oxide layer 4 is formed on the substrate 1, the bird's beak phenomena occurs in the nitride layer 3 at the defined active regions 1X, 1Y due to the presence of the field oxide layer, which cause the area of the active region to become relatively reduced. Accordingly, in order to solve such a problem, the width of the nitride layer 3 formed on the active regions 1X, 1Y should be determined by considering the size of the bird's beak 4X, 4Y which may occur during the forming process of the field oxide layer.
Further, in order to reduce the size of the bird's beak 4X, 4Y, a method for decreasing the thickness of the field oxide layer 4 has also been suggested. However, according to the above mentioned method, when the thickness of the field oxide layer 4 is reduced, the field oxide layer 4 can not accomplish its inherent separation function. Such a loss in function causes leakage current to be occurred between devices, when in use a strong electric field is applied to the surroundings of the field oxide layer 4.
Still furthermore, in order to reduce the size of the bird's beak 4X, 4Y, a method for increasing the thickness of the nitride layer 3, instead of decreasing the thickness of the substrate oxide layer 2 on the silicon substrate 1, has also been suggested. However, according to the above mentioned method, when mechanical stress is applied to the substrate, the crystallization structure of the substrate can be destroyed which results in the leakage current between devices, as mentioned above.
Therefore, according to the prior art, when a substrate oxide layer and a nitride layer are used, the nitride layer is selectively removed by a lithographic-etching process that utilizes the separation mask pattern to expose a portion of the substrate oxide layer. A field oxide layer is then grown both on the exposed substrate oxide layer and into the substrate, as mentioned above. The resulting field oxide layer is then grown to the certain extent on a portion of the silicon substrate, which underlies the nitride layer. This results in the undesirable formation of the bird's beak shape in the nitride layer positioned at the active region. As a result, the width of unit cell becomes excessively large.
Therefore, it is an object of the present invention to solve the problems presented in the prior art and to provide a method for forming a field oxide layer wherein an oxide layer for field oxide layer is deposited on the entire surface of the resulting structure, and, by utilizing a photoresist layer of the separation mask pattern, a field oxide layer is formed by lithographically etching the oxide layer for field oxide layer, utilizing the separation mask pattern.
The preceding objects should be construed as merely presenting a few of the more pertinent features and applications of the invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to both the summary of the invention and the detailed description, below, which describe the preferred embodiment in addition to the scope of the invention defined by the claims considered in conjunction with the accompanying drawings.